This invention relates to the addressing of matrix array type ferroelectric liquid crystal cells.
Hitherto dynamic scattering mode liquid crystal cells have been operated using a d.c. drive or an a.c. one, whereas field effect mode liquid crystal devices have generally been operated using an a.c. drive in order to avoid performance impairment problems associated with electrolytic degradation of the liquid crystal layer. Almost all of these devices have employed liquid crystals that do not exhibit ferroelectricity, and the material interacts with an applied electric field by way of an induced dipole. As a result they are not sensitive to the polarity of the applied field, but respond to the applied RMS voltage averaged over approximately one response time at that voltage. There may also be frequency dependence as in the case of so-called two-frequency materials, but this only affects the type of response produced by the applied field.
In contrast to this, a ferroelectric liquid crystal exhibits a permanent electric dipole, and it is this permanent dipole which will interact with an applied electric field. Ferroelectric liquid crystals are of interest in display, switching and information processing applications because they are expected to show a greater coupling with an applied field than that typical of a liquid crystal that relies on coupling with an induced dipole, and hence ferroelectric liquid crystals are expected to show a faster response. A ferroelectric liquid crystal display mode is described for instance by N. A. Clark et al in a paper entitled `Ferro-electric Liquid Crystal Electro-Optics Using the Surface Stabilized Structure` appearing in Mol. Cryst. Liq. Cryst. 1983 Volume 94 pages 213 to 234.
A particularly significant characteristic peculiar to ferroelectric smectic cells is the fact that they, unlike other types of liquid crystal cell, are responsive differently according to the polarity of the applied field. This characteristic sets the choice of a suitable matrix-addressed driving system for a ferroelectric smectic into a class of its own. A further factor which can be significant is that, in the region of switching times of the order of a microsecond, a ferroelectric smectic typically exhibits a relatively weak dependence of its switching time upon switching voltage. In this region the switching time of a ferroelectric may typically exhibit a response time proportional to the inverse square of applied voltage or, even worse, proportional to the inverse single power of voltage. In contrast to this, a (non-erroelectric) smectic A device, which in certain other respects is a comparable device, exhibits in a corresponding region of switching speeds a response time that is typically proportional to the inverse fifth power of voltage. The significance of this difference becomes apparent when it is appreciated first that there is voltage threshold beneath which a signal will never produce switching however long that signal is maintained; second that for any chosen voltage level above this voltage threshold there is a minimum time t.sub.S for which the signal has to be maintained to effect switching; and third that at this chosen voltage level there is a shorter minimum time t.sub.P beneath which the application of the signal voltage produces no persistent effect, but above which, upon removal of the signal voltage, the liquid crystal does not revert fully to the state subsisting before the signal was applied. When the relationship t.sub.S =f(V) between V and t.sub.S is known, a working guide to the relationship between V and t.sub.P is often found to be given by the curve t.sub.P =g(V) formed by plotting (V.sub.1,t.sub.2) where the points (V.sub.1,t.sub.1 and V.sub.2,t.sub.2) lie on the t.sub.S =f(V) curve, and where t.sub.1 =10t.sub.2. Now the ratio of V.sub.2 /V.sub.1 is increased as the inverse dependence of switching time upon applied voltage weakens, and hence, when the working guide is applicable, a consequence of weakened dependence is an increased intolerance of the system to the incidence of wrong polarity signals to any pixel, that is signals tending to switch to the `1` state a pixel intended to be left in the `0` state, or to switch to the `0` state a pixel intended to be left in the `1` state.
Therefore, a good drive scheme for addressing a ferroelectric liquid crystal cell must take account of polarity, and may also need to take particular care to minimise the incidence of wrong polarity signals to any given pixel whether it is intended as `1` state pixel or a `0` state one. Additionally, the waveforms applied to the individual electrodes by which the pixels are addressed need to be charge-balanced, at least in the long term. If the electrodes are not insulated from the liquid crystal, this is so as to avoid electrolytic degradation of the liquid crystal brought about by a net flow of direct current through the liquid crystal. On the other hand, if the electrodes are insulated, it is to prevent a cumulative build up of charge at the interface between the liquid crystal and the insulation.